Vehicle hydraulic braking system with an active simulator

ABSTRACT

Hydraulic braking apparatus for a motor vehicle, comprising: a service braking system (A), fed by a central hydraulic unit ( 3 ); an emergency braking system (B); a hand-control member (D,  16 ); a master cylinder ( 17 ); at least one safety valve ( 26, 28 ); a simulator (M), intended to resist the forward motion of the hand-control member (D,  16 ) with a reaction corresponding to the progress of a braking operation, such simulator comprising a cylinder ( 30 ) in which a simulator piston ( 31 ) slides while being subjected, in one direction, to a fluid pressure coming from the master cylinder ( 17 ) and, in the opposite direction, to a counterforce dependent on the travel of the hand-control member; admission solenoid valves ( 9   a - 9   d ) and exhaust solenoid valves ( 14   a - 14   d ), connected to the wheel brakes ( 2   a - 2   d ); sensors ( 8, 13   a - 13   d   , 24, 29 ) for the detection of various braking parameters; and a computer (C) capable of controlling the solenoid valves. The counterforce within the simulator (M) results from the action, on a surface of the simulator piston ( 31 ), of a modulated pressure, which comes from the fluid pressure supplied by the central hydraulic unit ( 3 ), and is controlled by the computer (C) in accordance with a determined law, alterable ad lib.

[0001] This invention relates to a hydraulic braking apparatus of thetype comprising, for the actuation of the wheel brakes:

[0002] a service braking system, supplied with a pressure brake fluid bya central hydraulic unit, using an external energy source;

[0003] an emergency braking system, controlled by muscular energy;

[0004] a hand-control member, the forward travel of which actuates theservice braking system or, in the case of a failure of the latter, theemergency braking system;

[0005] a master cylinder having at least one primary piston, the strokeof which is controlled by the hand-control member;

[0006] at least one safety valve, capable either of separating themaster cylinder from the wheel brakes when the service braking systemoperates properly or, should the service braking system fail to operatecorrectly, of connecting the master cylinder with at least one wheelbrake;

[0007] a feeling simulator, intended to resist the forward motion of thehand-control member with a reaction corresponding to the progress of abraking operation, such simulator comprising a cylinder in which asimulator piston may slide while being subjected, in one direction, to afluid pressure from the master cylinder and, in the opposite direction,to a counterforce dependent on the travel of the hand-control member;

[0008] pressure-fluid admission solenoid valves and exhaust solenoidvalves, connected to the wheel brakes;

[0009] sensors for the detection of various braking parameters, inparticular the travel of the hand-control member, and the pressures atvarious spots of the apparatus;

[0010] and a computer, connected to the various sensors and capable ofcontrolling the solenoid valves so as to obtain the desired pressures inthe wheel brakes.

[0011] A braking apparatus of said type is known, for instance, from FR2 772 706 or from U.S. Pat. No. 5,544,948.

[0012] In such an apparatus, in the course of a trouble-free operationin the service braking mode, the master cylinder is isolated and thefluid, contained in the master cylinder, cannot flow back to the wheelbrakes. The hand-control member, e.g. a brake pedal or a handbrakelever, retains a normal actuating travel, dependent on the exertedforce, thanks to the feeling simulator, which comprises a cylinderconnected to the master cylinder for the fluid transfers.

[0013] The well-known apparatuses operate satisfactorily and, besides,they make it possible to lay down a law of variation for the force to beapplied to the hand-control member as a function of the travel, whichmay give the driver a feeling like that he would get if the pressureinside the wheel brakes resulted directly from the pressure supplied bythe master cylinder, and from the muscular force exerted on the brakepedal.

[0014] Yet, in these apparatuses known per se, the law of variationconcerning the force to be applied to the hand-control member issomewhat fixed, and it cannot be altered in a simple and rapid manner.

[0015] Now then, on various grounds, more especially depending on thetype of the motor vehicle concerned, it is most desirable that said lawof variation should be alterable as simply and as rapidly as possible.

[0016] Besides, it is most desirable that the simulator should consumeas little fluid as possible, so that the emergency braking, achievedwith the help of the master cylinder, may remain as efficient aspossible.

[0017] Therefore, the primary object of the present invention is toprovide a hydraulic braking apparatus, which meets the variousabove-mentioned requirements better still than currently and which, moreparticularly, makes it possible to alter, in an easy and rapid manner,the law of variation for the force to be applied to the hand-controlmember, as a function of the travel.

[0018] Moreover, it is to be desired that the solution, as it isprovided herein, be implemented in a comparatively simple and especiallyreliable way.

[0019] According to the invention, a hydraulic braking apparatus for amotor vehicle, of the above-defined kind, is characterised in that thecounterforce within the simulator results from the action, on a surfaceof the simulator piston, of a modulated pressure, which comes from thefluid pressure supplied by the central hydraulic unit and is controlledby the computer in accordance with a determined law, alterable ad lib,as a function of the pedal travel.

[0020] Any law of variation whatever, as regards the force exerted onthe hand-control member as a function of the travel, may be programmedin the computer, without having to modify the apparatus in another way.

[0021] Preferably, the surface of the simulator piston, which is underthe modulated pressure, defines a variable-capacity chamber connected inparallel to an admission solenoid valve, for the pressure fluid suppliedby the central hydraulic unit, and to an exhaust solenoid valve,connected to the feed tank, the opening and closing of said solenoidvalves being controlled by the computer so that the pressure inside thesimulator chamber may follow the desired law.

[0022] In a preferred manner, the solenoid valves, connected to thesimulator chamber, are of the “on/off” type and the pressure dropbetween the inlets and the outlets of the solenoid valves may be lineardependently on the control current strength.

[0023] Advantageously, in the simulator, the counterforce is theresultant of a resilient force acting upon the simulator piston in theopposite direction to the fluid pressure coming from the mastercylinder, and of a variable force resisting the resilient force, suchvariable force being generated by the modulated pressure acting on asurface of the simulator piston.

[0024] The resilient force may be produced by at least one resilientreturn means. In a preferred manner, such resilient return meanscomprises an air spring.

[0025] The cylinder of the simulator may comprise two intercommunicatingcoaxial bores with different diameters, and a stepped piston including asmall-diameter portion sliding in a leakproof manner inside thesmall-diameter bore, and a greater-diameter portion sliding in aleakproof manner in the large-diameter bore, the end wall of thesmall-diameter bore comprising a port connected to the master cylinderfor the fluid pressure from the master cylinder to be applied to thesmall cross-section of the stepped piston, whereas an annular chamber isdefined between the transition wall of the bore and the largecross-section of the stepped piston, such annular chamber beingconnected in parallel to the respective admission and exhaust solenoidvalves.

[0026] The end wall of the simulator cylinder, which closes the largecross-section bore in the opposite direction to the small cross-sectionbore, may comprise an opening for the passage of a rod resting on thelarge cross-section of the stepped piston and exerting the resilientforce on said piston. Such rod may be integral with a pneumatic pistonsliding inside a pneumatic cylinder, for its part integral with thesimulator cylinder, such pneumatic cylinder being connected to anexternal air-pressure source, intended e.g. for a pneumatic suspension.

[0027] A nonreturn valve may be provided on a pressure-air lineconnected to the pneumatic cylinder, such valve allowing the inflow ofthe pressure air into the cylinder and opposing its outflow.

[0028] A mechanical compression spring may be arranged inside thepneumatic cylinder to as to act upon the pneumatic piston in the samedirection as the air pressure.

[0029] In addition to the above-mentioned arrangements, the inventionprovides a number of arrangements as well, which will be more fullyexplained in the following detailed description of an embodiment of thepresent invention, by way of example and by no means as a limitation,when taken in conjunction with the accompanying drawings.

[0030]FIG. 1 is a schematic illustration of a hydraulic brakingapparatus according to the present invention;

[0031]FIG. 2 is a simplified diagram on a larger scale of the simulatorand of the master cylinder;

[0032]FIG. 3 illustrates an example of a law of variation for the forceto be exerted on the hand-control member as a function of the travel,and of the modulated pressure; and

[0033]FIG. 4 shows the variation of the air pressure inside thepneumatic cylinder as a function of the piston stroke.

[0034]FIG. 1 shows a hydraulic braking apparatus 1 intended for a motorvehicle and devised to actuate the wheel brakes 2 a and 2 b for thefront wheels, and 2 c and 2 d for the rear wheels. In a conventionalmanner, each wheel brake comprises a cylinder in which a piston iscapable of moving under the action of a pressure fluid, so as to apply abrake pad or shoe against an element, either a disk or a drum,rotationally locked with the wheel to be braked.

[0035] The apparatus 1 comprises a service braking system A, suppliedwith a pressure fluid by a central hydraulic unit 3, using an externalenergy source, and an emergency braking system B, controlled by muscularenergy.

[0036] The central hydraulic unit 3 comprises a pump 4 driven by a motor5, e.g. an electric motor. The pump 4 delivers pressure fluid to a mainsupply line 6, on which a hydropneumatic accumulator 7 is mounted. Apressure sensor 8, which outputs an electrical quantity indicative ofthe pressure value in the line 6, is also fitted on said line. The inletside of the pump 4 is connected to a nonpressure fluid tank 9, alsocalled a feed tank.

[0037] The pressure fluid line 6 is connected in parallel, throughsolenoid valves, 9 a, 9 b, 9 c and 9 d, to the respective wheel brakes 2a-2 d. Such solenoid valves are two-position ones, i.e. they are eitheropen or closed, and they are pilot-controlled by means of a programmablecomputer, or microprocessor C. For a better and clearer understanding ofthe drawings, the electrical connections between the control coils ofthe solenoid valves and the computer C are represented by the beginningof a line only.

[0038] In the rest position, the valves 9 a-9 d are closed, as shown inFIG. 1. The outlet of a valve 9 a, 9 b is connected to a front wheelbrake 2 a, 2 b through a hydraulic separator, 10 a and 10 brespectively. The outlets of the valves 9 c and 9 d are directlyconnected to the rear wheel brakes 2 c, 2 d. A pressure-equalizing valve11 is intercalated between the outlets of the valves 9 a and 9 b, andanother pressure-equalizing valve 12 is intercalated between the outletsof the valves 9 c, 9 d as well.

[0039] A pressure sensor 13 a, 13 b, 13 c, 13 d is fitted on each supplyline for the brakes 2 a-2 d so as to output an electrical quantityrepresentative of the applied pressure. The outputs of these sensors 13a-13 d are connected by lines (not shown) to the computer C. The outputof the sensor 8 is also connected to C.

[0040] The exhaust solenoid valves 14 a, 14 b, 14 c and 14 d areconnected in parallel to the admission solenoid valves 9 a-9 d, on thelines which are connected to the inlets to the wheel brakes. Said valves14 a-14 d are two-position ones, i.e. either an open or a closedposition, and they are connected to a line 15 returning the fluid to thefeed tank 9. At rest, the valves 14 a-14 d are open, as shown in FIG. 1.

[0041] The exhaust valves 14 a-14 d are also pilot-controlled by thecomputer C, which comprises outputs connected to each coil controllingthe valves 14 a-14 d.

[0042] The apparatus comprises a hand-control member D, generallyconsisting of a brake pedal 16 and a master cylinder 17, in which aprimary piston 18 and a secondary piston 19 may slide, both of themhaving the same cross-section S1. The pedal 16 is connected to thepiston 18 by a rod 20, linked to the pedal. Herein, a “forward travel ormotion” designates a motion of the pedal 16 towards the master cylinder17, which brings about a travel of the piston 18 towards the secondarypiston 19 and the opposite end wall of the cylinder 17.

[0043] A primary chamber 21, filled with fluid, is defined between thepiston 18 and the piston 19, and a spring 21 a is disposed in saidchamber between both pistons. A secondary chamber 22, filled with fluidtoo, is defined between the piston 19 and the end wall of the mastercylinder 17, remote from the piston 18. A spring 22 a is arranged in thechamber 22.

[0044] An electric contact 23, sensitive to the forward travel of thepedal 16, is provided in a conventional manner for the control of the“stop” lights. A terminal of this contact 23 is connected to a terminalof the computer C, which actuates the service braking system A inresponse to the forward travel of the brake pedal 16. Besides, a strokesensor 24 for the pedal 16 is provided and it transmits a correspondingelectrical signal to another input terminal of the computer C. Forinstance, the sensor 24 may output data concerning the amplitude of thetravel of the pedal 16 as well as data regarding the travel speed.

[0045] Both chambers 21, 22 of the master cylinder are connected to thefeed tank 9 through a nonreturn valve (not shown), which enables thechambers 21, 22 to be supplied with fluid but precludes a backflow.

[0046] The primary chamber 21 is connected with the wheel brake 2 b, bymeans of a pipe 25 fitted with a safety or stop solenoid valve 26. Thesolenoid valve 26 is controlled by the computer C and it is of thetwo-position type, i.e. open or closed; it is in the open position whenthe apparatus is at rest.

[0047] The chamber 22 is connected to the wheel brake 2 a through a line27 and a solenoid valve 28. A pressure sensor 29 is fitted on the line27, between the master cylinder 17 and the solenoid valve 28. The sensor29 outputs an electrical signal, which is applied through a link (notshown) to an input of the computer C.

[0048] Besides, the braking apparatus 1 comprises a brake-actuationsimulator M, intended to resist the forward motion of the brake pedal 16with a reaction corresponding to the progress of a braking operation.

[0049] Such simulator M comprises a cylinder 30 (cf. FIGS. 1 and 2), inwhich a simulator piston 31 slides.

[0050] The cylinder 30 comprises two intercommunicating coaxial bores 30a, 30 b with different diameters. The smaller-diameter bore 30 a isbounded, in the opposite direction to the bore 30 b, by a wall 30 c inwhich a central port 32 is provided. Such port 32 is connected through apipe 33 with one chamber of the master cylinder 17, in the presentinstance the secondary chamber 22.

[0051] The piston 31 is a stepped piston including a small-diameterportion 31 a with a cross-section S2, sliding in a leakproof mannerinside the bore 30 a, and a greater-diameter portion 31 b sliding in aleakproof manner in the bore 30 b. The portion 31 b is edged with acylindrical skirt, the concave side of which is in the oppositedirection to the bore 30 a.

[0052] An annular chamber 34, with a cross-section S3, is formed betweenthe portion 31 b and the transition wall 30 d situated between the bores30 a and 30 b. Said annular chamber 34 surrounds the portion 31 a, andits capacity varies as a function of the position of the piston 31 alongthe axis of the cylinder 30. A port 35 is provided in the wall of thecylinder 30 and it opens into the bore 30 b, near the wall 30 d, whichis the end wall of the chamber 34.

[0053] The port 35 is connected in parallel (FIG. 1) to a pressure-fluidadmission solenoid valve 36 and to an exhaust solenoid valve 37. Thesolenoid valves 36 and 37 are of the “on/off” type, which means thatthey have two positions and that they are either open or closed.Preferably, the pressure drop between the inlets and the outlets of thesolenoid valves 36, 37 follows a linear variation, as a function of thecontrol current strength for these valves. The control coils of thevalves 36, 37 are connected by electrical lines 38 and 39 to twoterminals of the computer C. The inlet of the valve 36 is connected tothe pressure-fluid supply line 6 coming from the central hydraulic unit3. The outlet of the valve 37 is connected to the fluid-return line 15leading to the tank, or feed tank 9.

[0054] The computer C controls the valves 36 and 37 as a function of thetravel of the pedal 16 so as to obtain a modulated pressure Pehb, whichis applied within the annular chamber 34 and exerted on the annularsurface S3 of the piston 31.

[0055] The end wall 30 e, closing the bore 30 b in the oppositedirection to the bore 30 a, comprises an opening 40 for the passage of arod 41, which is coaxial with the cylinder 30 and bears on the piston31. The rod 41 is integral with a pneumatic piston 42 (i.e. a pistonsubjected to a gas pressure), disposed inside a cylinder 43 coaxial withthe cylinder 30 and attached to the latter. Generally, the diameter ofthe piston 42 is greater than the diameter of the portion 31 b of thepiston 31. These diameters are determined so as to achieve the desiredforces, while taking into account the pressures involved. The rod 41traverses the end wall of the cylinder 43.

[0056] The chamber 44 of the cylinder 43, located on the rod 41 side, isconnected to the atmosphere through at least one port, not visible inthe drawings. In the same way, the chamber 45 in the bore 30 b, whichreceives the rod 41, is connected to the atmosphere through at least oneport, not visible on the drawings either.

[0057] In the opposite direction to the rod 41, the piston 42 has across-section S4 and it defines, inside the cylinder 43, a chamber 46with the same cross-section S4. Such chamber 46 is connected, via a port47 provided in the end wall remote from the cylinder 30, to a pipe 48,for its part connected to an external air pressure source 49. Moreparticularly, the source 49 may be a compressed-air source for apneumatic suspension. By way of a non-limiting example, thecompressed-air pressure, supplied by the pipe 48, may be in the order of10 bars.

[0058] A nonreturn valve 50 is provided on the pipe 48, near the port47, so as to allow the inflow of the pressure air from the pipe 48 intothe chamber 46 and to prevent an air flow in the reverse direction.

[0059] A compression spring 51 is arranged in the chamber 46, betweenthe piston 42 and the end wall of the chamber, for an action in the samedirection as the air pressure. Such spring 51 exerts on the piston 42but a negligible return force, compared with the forces generatedthrough the pressures.

[0060] Thus, in one direction, the piston 31 of the simulator is underthe fluid pressure coming from the master cylinder 17 and exerted on thesmall portion 31 a and, in the opposite direction, it is subjected to acounterforce, which depends on the travel of the pedal 16. Suchcounterforce is equal to the difference between the resilient force,applied by the piston 42 and transmitted through the rod 41, and thevariable force, which is exerted by the modulated pressure Pehb on thecross-section S3 of the stepped piston 31.

[0061] The simulator M takes action when the service braking systemoperates in a trouble-free manner. Under those circumstances, the valves28 and 26 are closed so that the fluid within the chamber 21 is confinedin a constant volume; as a matter of fact, the pressure prevailing insaid chamber 21 is the same as that existing in the chamber 22,connected to the pipe 33.

[0062]FIG. 2 is a simplified diagram, which makes it possible to laydown relationships between the various quantities. The variousparameters are designated as follows:

[0063] Frod=force exerted by the pedal 16 on the rod 20

[0064] Pmc=pressure inside the master cylinder 17

[0065] S1=cross-section of the master cylinder 17

[0066] Xt=travel of the rod 20 and of the piston 18

[0067] S2=cross-section of the portion 31 a

[0068] Xsimu=travel of the piston 31

[0069] S3=cross-section of the annular chamber 34

[0070] Pehb=modulated pressure at the port 35

[0071] S4=cross-section of the piston 42, on the chamber 46 side

[0072] P0=initial pressure in the chamber 46

[0073] v0=initial volume of the chamber 46

[0074] h0=initial axial length of the chamber 46.

[0075] In the absence of fluid leaks:

S1.Xt=S2.Xsimu, whence Xt=(S2/S1).Xsimu.

[0076] This being so, the mode of operation of the apparatus is asfollows.

[0077] At rest, that is when the pedal 16 is not depressed, the variousconstitutive parts of the apparatus are in the positions illustrated inFIG. 1.

[0078] As soon as the pedal 16 is actuated, the contact 23 sends thecomputer C data indicating the beginning of a braking operation. Thecomputer C causes the closure of the valves 26 and 28, thus separatingthe master cylinder 17 from the brakes 2 a, 2 b of the front wheels.Besides, the computer C controls the solenoid valves 9 a-9 d and 14 a-14d so as to induce, in the wheel brakes 2 a-2 d, a pressure which is afunction of the travel of the pedal 16, more particularly a function ofthe position and the travel speed of the latter. Other factors, e.g. thedetection of a wheel locking may be taken into consideration by thecomputer C so as to act on the brake pressure.

[0079] Moreover, the computer C controls the valves 36, 37 in order toobtain, at the inlet 35, a modulated control pressure Pehb, which variesaccording to a predetermined law, as a function of the pedal travel.

[0080] The curve L1 represented in FIG. 3 is an example of a law ofvariation for the pressure Pehb, the values of which are indicated bythe bar-graduated scale along the Y-axis on the right-hand side, as afunction of the pedal travel expressed in millimeters along the X-axis.

[0081] When the piston 42 travels in the direction meaning an increasedvolume of the chamber 46, it is under an air pressure, equal to thatsupplied by the line 48. But, when the piston 42 moves in the directionresulting in a reduced capacity of the chamber 46, the valve 50 closesand the air volume confined inside the chamber 46 undergoes acompression process, such a compression being generally considered asadiabatic, so that the air pressure rises in the chamber 46.

[0082] The force, exerted by the pressure Pehb on the cross-section S3of the piston 31, is subtracted from the force applied by the piston 42.The pressure Pmc of the master cylinder, applied to the cross-section S2of the portion 31 a, balances such difference. Said pressure Pmc,applied to the piston 18 of the master cylinder, generates the reaction,resisting the forward travel of the pedal 16.

[0083] The pressure inside the chamber 46 being designated by Px, for anaxial length (h0−Xt) of this chamber, the expression may read:

Frod=Pmc.S2=(Px.S4)−(Pehb.S3)

[0084] and the various quantities may be inferred from the relationshipexisting between the pressure Px and the volume of the air mass confinedin the chamber 46.

[0085] The variation of the force Ft to be exerted on the rod 20 as afunction of the travel of such rod is illustrated by the curve G1 inFIG. 3, the values of the force Ft being indicated by thenewton-graduated scale along the Y-axis, on the left-hand side.

[0086] The law L1 controlling the pressure Pehb may be altered ad lib,through a programming of the computer C. It means that the curve G1 maybe altered ad lib too, without having to modify the equipment for allthat.

[0087] At the beginning of the travel of the pedal 16, the force to beexerted on the rod 20 should not be too high, with the result that thepressure Pehb is comparatively high in the case of short travels, so asto reduce the force to be exerted on the pedal 16.

[0088] The more the pedal 16 is depressed, the more the chamber 22 feedsfluid to the bore 30 a. The piston 31 travels towards the cylinder 43while pushing back the rod 41 and the piston 42. The volume of air,confined inside the chamber 46, exerts an increasing pressure, whichresults in a greater force to be applied to the rod 20. The pressurePehb is decreasing from a certain value of the travel of the pedal 16onwards, so that the resistance to the forward travel may be greatenough towards the end of the travel.

[0089] At that time, the driver actually “feels” the level of thebraking force applied by an external energy source, irrespective of hismuscular effort.

[0090]FIG. 4 shows in a curve K1 the variation of the air pressure Px(in the chamber 46) expressed in bars along the Y-axis, as a function ofthe piston 42 travel expressed in millimeters along the X-axis.

[0091] Should some trouble happen in the service braking system, thecalculator C would detect this failure, e.g. on the basis of too low apressure value, output by the sensors 13 a-13 d, though the pedal 16 hasmoved.

[0092] As a consequence of it, the computer C keeps the valves 26, 28 intheir open positions, with the result that pressure fluid, coming fromthe master cylinder 17, may flow via two independent circuits towardsthe brakes 2 a and 2 b, thus making it possible to carry out anemergency braking operation.

[0093] Moreover, the fluid contained in the bore 30 a is driven underthe action of the piston 42 still under the air pressure, and apressure, substantially higher than that prevailing inside the chamber22 of the master cylinder, is produced in the bore 30 a. Therefore, thepiston 31 is pushed back, to the left-hand side in FIG. 1, and itdischarges some fluid into the line 27 supplying the brake 2 a, theeffect of it being that the braking requirements are met still better.

[0094] As a matter of fact, in the presence of a failure in the servicebraking system, the emergency braking system, based on the muscularenergy, must enable the driver to apply the brakes with a sufficientdeceleration which is, for the time being, fixed at to 0.3 g, inresponse to a determined force, e.g. 500 newtons (500 N), exerted on thebrake pedal 16. Owing to the fact that the fluid volume, coming from thebore 30 a of the simulator, is recovered for the emergency braking, suchan emergency braking operation can be ensured even in the case of acomparatively heavy motor vehicle, e.g. in the order of 4.000 kg.

We claim:
 1. A hydraulic braking apparatus intended for a motor vehicleand comprising, for the actuation of the wheel brakes: a service brakingsystem (A), supplied with a pressure brake fluid by a central hydraulicunit (3), using an external energy source; an emergency braking system(B), controlled by muscular energy; a hand-control member (D, 16), theforward travel of which actuates the service braking system or, in thecase of a failure of the latter, the emergency braking system; a mastercylinder (17) having at least one primary piston (18), the stroke ofwhich is controlled by the hand-control member at least one safety valve(26, 28), capable either of separating the master cylinder (17) from thewheel brakes (2 a, 2 b) when the service braking system operatesproperly or, should the service braking system fail to operatecorrectly, of connecting the master cylinder with at least one wheelbrake; a feeling simulator (M), intended to resist the forward motion ofthe hand-control member (D, 16) with a reaction corresponding to theprogress of a braking operation, such simulator comprising a cylinder(30) in which a simulator piston (31) may slide while being subjected,in one direction, to a fluid pressure coming from the master cylinder(17) and, in the opposite direction, to a counterforce dependent on thetravel of the hand-control member; pressure-fluid admission solenoidvalves (9 a-9 d) and exhaust solenoid valves (14 a-14 d), connected tothe wheel brakes; sensors (8, 13 a-13 d, 24, 29) for the detection ofvarious braking parameters, in particular the travel of the hand-controlmember, and the pressures at various spots of the apparatus; and acomputer (C), connected to the various sensors and capable ofcontrolling the solenoid valves so as to obtain the desired pressures inthe wheel brakes, characterised in that the counterforce within thesimulator (M) results from the action, on a surface (S3) of thesimulator piston (31), of a modulated pressure (Pehb), which comes fromthe fluid pressure supplied by the central hydraulic unit (3) and iscontrolled by the computer (C) in accordance with a determined law (L1),alterable ad lib as a function of the pedal travel.
 2. The apparatusaccording to claim 1, characterised in that the surface (S3) of thesimulator piston, which is under the modulated pressure (Pehb), definesa variable-capacity chamber (34) connected in parallel to an admissionsolenoid valve (36), for the pressure fluid supplied by the centralhydraulic unit (3), and to an exhaust solenoid valve (37), connected tothe feed tank (9), the opening and closing of said solenoid valves (36,37) being controlled by the computer (C) so that the pressure (Pehb)inside the simulator chamber (34) may follow the desired law.
 3. Theapparatus according to claim 2, characterised in that the solenoidvalves (36, 37), connected to the chamber (34) of the simulator, are ofthe “on/off” type.
 4. The apparatus according to claim 3, characterisedin that the pressure drop between the inlets and the outlets of thesolenoid valves (36, 37) is linear dependently on the control currentstrength.
 5. The apparatus according to claim 4, characterised in thatthe cylinder (30) of the simulator comprises two intercommunicatingcoaxial bores (30 a, 30 b) with different diameters, and a steppedpiston (31) including a small-diameter portion (31 a) sliding in aleakproof manner inside the small-diameter bore (30 a), and agreater-diameter portion (31 b) sliding in a leakproof manner in thelarge-diameter bore (30 b), the end wall (30 c) of the small-diameterbore comprising a port (32) connected to the master cylinder for thefluid pressure from the master cylinder to be applied to the smallcross-section (S2) of the stepped piston, whereas an annular chamber(34) is defined between the transition wall (30 d) of the bore and thelarge cross-section of the stepped piston, such annular chamber beingconnected in parallel to the respective admission and exhaust solenoidvalves (36, 37).
 6. The apparatus according to claim 5, characterised inthat, in the simulator (M), the counterforce is the resultant of aresilient force acting upon the simulator piston (31) in the oppositedirection to the fluid pressure coming from the master cylinder (17),and of a variable force resisting the resilient force, such variableforce being generated by the modulated pressure (Pehb) acting on asurface (S3) of the simulator piston.
 7. The apparatus according toclaim 6, characterised in that the resilient force is produced by atleast one resilient return means.
 8. The apparatus according to claim 7,characterised in that the resilient return means comprises an air spring(42, 43).
 9. The apparatus according to claim 6, characterised in thatthe end wall (30 e) of the simulator cylinder, which closes the largecross-section bore in the opposite direction to the small cross-sectionbore, comprises an opening (40) for the passage of a rod (41) resting onthe large cross-section (31 b) of the stepped piston (31) and exertingthe resilient force on said piston.
 10. The apparatus according to claim9, characterised in that said rod (41) may be integral with a pneumaticpiston (42) sliding inside a pneumatic cylinder (43), for its partintegral with the simulator cylinder (30), said pneumatic cylinder beingconnected to an external air-pressure source (49) intended, moreparticularly, for a pneumatic suspension.
 11. The apparatus according toclaim 10, characterised in that a nonreturn valve (50) is provided on apressure-air line (48) connected to the pneumatic cylinder, said valveallowing the inflow of the pressure air into the cylinder and opposingits outflow.
 12. The apparatus according to claim 11, characterised inthat a mechanical compression spring (51) is arranged inside thepneumatic cylinder to as to act upon the pneumatic piston in the samedirection as the air pressure.